A common problem associated with sensorineural hearing loss is recruitment. A hearing impaired person suffering from recruitment has an elevated threshold for soft sounds. This means that soft sounds which are audible to a person with normal hearing will have to be made louder in order to be heard by the hearing impaired person. However, with recruitment, loud sounds may be just as loud for the hearing impaired person as for the person with normal hearing. This represents a loss of dynamic range for the hearing impaired. This loss of dynamic range may vary with frequency. For example, at low frequencies the hearing impaired person may have nearly the same dynamic range as the person with normal hearing, but at high frequencies the dynamic range of the hearing impaired person may be considerably reduced. This impaired dynamic range is often referred to as the residual dynamic range.
The loss of dynamic range in the hearing impaired is most often attributed to malfunction of the outer hair cells of the cochlea. Sound vibrations in the air are transmitted from the ear drum and through the ossicles of the middle ear to the inner ear and the cochlea. Inside the cochlea are the flexible tectorial membrane and the more rigid basilar membrane. Between these two membranes lie the inner and outer hair cells. Ninety-five percent of the afferent neural fibers which transmit acoustic information to the brain are connected to the inner hair cells. The longest cilia of the outer hair cells are connected to the tectorial membrane, but the inner hair cells have no such connection. Both the inner and outer hair cells are connected to the basilar membrane through supporting cells. Vibrations passing between the tectorial and basilar membranes cause more motion in the flexible tectorial membrane than in the basilar membrane. This difference in motion causes a sheering motion along the outer hair cells. The outer hair cells react to this shearing motion in a complex manner. The entire mechanism is not yet clearly understood but it appears that the outer hair cells stretch and contract according to the intensity of the vibrations in a manner which amplifies these vibrations. For larger amplitude vibrations, however, the outer hair cell motion saturates causing a reduction in amplification. This nonlinear, saturating amplification corresponds to a natural dynamic range compression. The compressed vibrations from the outer hair cells are communicated to the inner hair cells and then through the afferant neural fibers to the brain. When the outer hair cells malfunction, there is a loss of natural compression and recruitment occurs. The inner hair cells may continue to functions normally and there may be a mild to moderate hearing loss. More severe hearing losses will occur with loss of inner hair cell function.
Many hearing aid instruments have been designed to deal with this problem. The approach taken is to compress the dynamic range of the input sound signal so that it more nearly fits into the residual dynamic range of the recruited ear. The ratio of input dynamic range in dB to compressor output dynamic range in dB is called the compression ratio. To adequately specify the compressor, the compression ratio needs to be accompanied by a static gain value. This static gain value will determine at which input power level the system delivers a specified fixed gain. For example the static gain may be set so that at 80 dB SPL input power, the system delivers unity gain. If the compressor is set to a 2:1 compression ratio, then at 60 dB SPL input power the system will produce a 70 dB SPL output, that is a gain of 10 dB, and at 100 dB SPL input power the system will produce a 90 dB SPL output, that is a gain of -10 dB.
Usually the compression ratio is not constant over the entire input power range. A low level compression knee may be defined. For input powers below this low level compression knee, the compression ratio may be 1:1, that is, a fixed linear gain may be applied. The designated compression ratio (e.g. 2:1) may take affect only for input power levels above this low level compression knee. A high level or limiting knee may also be defined. For input power levels above this high level knee, the compression ratio may increase or even become infinite, or it may be that the output level is fixed regardless of increase in input level. A system which has only a high level compression knee below which the compression ratio is 1:1 (linear gain) is called a limiter. A system which has a low level compression knee positioned at 40-50 dB SPL is termed a full range compressor.
Even without reference to the electro-mechanics of the inner ear and the natural loss of compression due to malfunction of the outer hair cells, the need for compressors or limiters in hearing aids has long been recognized. The need for hearing aids to have large gains to make softer sounds audible has driven amplifiers and output transducers out of their linear ranges. Earlier hearing aids accomplished limiting by letting the amplifier and/or output transducer clip. Unfortunately this caused harmonic distortion which, at high frequencies, masked softer speech sounds and generally reduced fidelity in the system (See M. C. Killian, The K-Amp Hearing Aid: An Attempt to Present High Fidelity for Persons with Impaired Hearing, American Speech-Language-Hearing Association, July 1993, at 52-74). Later systems introduced limiters to help alleviate this problem, and still later systems used full range dynamic range compression (See e.g. Fred Waldhauer et al., Full Dynamic Range Multiband Compression in a Hearing Aid, The Hearing Journal, September 1988, at 1-4).
The compression process requires a means for measuring the power of the input signal and generating a dynamically varying gain as a function of this input power. This gain is then applied to the signal which is delivered to the ear. When the input power is low, this gain will generally be high so that soft sounds are made louder. When the input power is high, this gain will generally be low so that loud sounds are not made too loud. The measure of input power requires averaging over time. The time span of the averaging defines a compression time constant. If the time span is very long then the compressor will react slowly to changes in input power level. This is sometimes referred to as Automatic Gain Control (AGC) where time constants of one to two seconds are typical. When the time span of the averaging is short the compressor will react quickly to changes in input power level. With a time span of approximately five to fifty milliseconds, the compressor may be referred to as a syllabic rate compressor. A syllabic rate compressor will limit the gain of a loud vowel sound while amplifying a soft consonant which immediately follows it.
In most designs there is both an attack and release compressor time constant. The attack time constant determines the time it takes for the compressor to react at the onset of a loud sound. That is, the time it takes to turn down the gain. The release time constant determines the time it takes for the system to turn up the gain again after the loud sound has terminated. Most often the attack time is quite short (&lt;5 milliseconds) with the release time being longer (anywhere from 15 to 100s of milliseconds).
Even with separate attack and release times, there have still been problems with compressor time constants. With a long release time, any short impulse in the room (e.g. the clank of a dish) will cause the gain to be shut down for the length of the relatively long release time. On the other hand, if the time constant is always short, it will cause an annoying swell in volume every time a speaker takes a breath. This problem has been alleviated by the introduction of adaptive time constants. Hotvet introduced in U.S. Pat. No. 4,718,499 an adaptive time constant system in which the release time constant for a loud sound in silence is short but the release time constant gradually becomes longer proportional to the length of the louder sounds in the environment. Thus, if a speaker speaks in a normal rhythm, the release time constant will grow longer, reducing the amplitude swell in the brief silences between words. Others have also discussed multiple time constant systems with a similar goal in mind (See e.g. R. F. Laurence, et al., A Comparison of Behind-the Ear High-Fidelity Linear Hearing Aids and Two-Channel Compression Aids, in the Laboratory and in Everyday Life, Br. J. Audiol., 1983, at 17:31-48; and Brian Moore, et al., Optimization of a Slow-Acting Automatic Gain Control System for Use in Hearing Aids, Br. J. Audiol., 1991, at 25:171-182).
To match the variability of recruitment with frequency, a compressor is often designed to perform differently in different frequency bands. A multi-band compressor divides the input signal into multiple frequency bands and then measures power in each band and compresses each band separately with possibly different compression ratios and time constants in the different bands. For example a properly designed two band compressor can make soft high frequency consonants audible while suppressing low frequency competing noises occurring simultaneously. Vilchur (See E. Vilchur, Signal Processing to Improve Speech Intelligibility in Perceptive Deafness, J. Acoust. Soc. Am. 53, 1973, at 1646-1657) discussed a bench top prototype of a two band compressor. Barfod (See J. Barfod, Multichannel Compression Hearing Aids, Report No. 11, The Acoustic Laboratory, Technical University of Denmark, 1976) discussed compressors of up to four bands. These compressors also had variable time constants in the different frequency bands.
The outer hair cells of the cochlea, when functioning normally, are often thought to perform compression function in overlapping frequency bands called critical bands. These frequency bands are spaced linearly at intervals of approximately 100 Hz at frequencies below about 500 Hz, and are spaced logarithmically at approximately third octave intervals above 500 Hz. Thus, the outer hair cells behave as a biological critical band compressor. The time constant associated with this compressor has been approximated to be about 1 ms. Lippman et. al. (See R. P. Lippman, et al., Study of Multichannel Amplitude Compression and Linear Amplification for Persons with Sensorineural Hearing Loss, J. Acoust. Soc. Am. 69(2), February 1981, at 524-534) designed a benchtop 16 band compressing hearing aid system with the bands tuned to match the critical bands of hearing. Each band represented a separate compression channel. Two settings of this compressor were compared against a linear non-compressing system. Martin (See G. R. Martin, Studies of Real-Time Multiband Adaptive Gain Hearing Aids, MIT, September 1992, at 1-103) discussed a 3.sup.rd octave band compression hearing aid system using digital signal processing.
As the number of compression bands increases, each with its own compression ratio and static gain, it is possible to view the compressor as having an almost continuously varying compression ratio as a function of frequency. In this case the system may, be represented as a set of frequency dependent gain curves. Each gain curve applies at a certain input power level. For input between these power levels, the system interpolates between gain curves. Killian (previously cited) discusses the K-amp hearing aid system which integrates power in one band but uses the power estimate to interpolate between low level and high level frequency response curves. The low power level frequency response curve has generally more gain and, in particular, more gain at high frequencies then at low. The high power level frequency response curve has generally less gain and is more flat across frequencies. There is an optional setting which allows the low power level curve to also be set flat.
The process of adjusting the compression ratios or gain curves of a compressor is central to the hearing aid fitting process. One approach to doing this is to attempt to adjust the compressor so that for all input levels and all frequencies the hearing impaired listener has the same impression of loudness that a normal listener would have. Loudness is a perceptual quantity which can under certain constraints be plotted as a function of input power level. The loudness growth curve may be measured by presenting a number of input signals at different levels and asking the listener to subjectively rate these on a perceptual scale (e.g. 1 to 10). By measuring the loudness growth curves of an impaired listener at different frequencies and comparing these to the loudness growth curves of an average of normal listeners, a loudness matching compression fitting can be attempted. To accurately match loudness growth curves, the hearing instrument would permit continuously variable compression ratio over input level. In this case it is more useful to think in terms of continuously variable input/output power curves. The system described above with low and high level compression knees is able to implement only three segment piecewise input output curves. Barfod (previously cited) and Lippman et. al. (previously cited) attempted to fit their multi-band compression systems so as to restore the loudness growth curves of the impaired ear to match those of the normal ear.
Loudness matching compression fitting has its limits. If the recruited ear has 5 dB of residual dynamic range it will not be effective to compress a 90 dB input dynamic range into this 5 dB. Instead, some amount of compression will be applied and then a static gain defined so that the most useful part of the input dynamic range (e.g. typical speech range) is roughly centered in the residual dynamic range. Limiting will be applied for louder signals. Finding good compromises in fitting compressors is central to the art of hearing aid fitting.
There has been some discussion about whether it is indeed necessary to test the loudness growth curves of the impaired listener as part of the fitting process or whether it is possible to predict them from the threshold audiograms. Kollmeier et al. (See B. Kollmeier, el al., Speech Enhancement by Filtering in the Loudness Domain, Acta Otolaryngol (Stockh) 1990, Suppl. 469:207-214) has shown that the shape of loudness growth curves becomes less predictable with increasing hearing loss. That is, the variance between subjects increases with hearing loss. This indicates that successful prediction from the threshold is unlikely.
There has been much discussion regarding the nature of improvements due to compression. Vilchur (previously cited) and Yanick (See P. Yanick, Effects of Signal Processing on the Intelligibility of Speech in Noise for Subjects Possessing Sensorineural Hearing Loss, J. Am. Audiol. Soc. 1, 1976, at 229-238) showed improvements in intelligibility with their compression systems, while Abramovits (See R. Abramovits, The Effects of Multichannel Compression Amplification and Frequency Shaping on Speech Intelligibility for Hearing Impaired Subjects, Unpublished doctoral thesis, City University of New York, 1979), Mangold et al. (See S. Mangold, et al., Programmable Hearing Aid with Multichannel Compression, Scand. Audiol. 8, 1979, at 121-126), O'Loughlin (See B. O'Loughlin, Evaluation of a Three Channel Compression Amplification System on Hearing-Impaired children, Aust. J. Audiol. 2, 1980, at 1-9), and Lippman et al. (previously cited) failed to show intelligibility improvements. It has also been argued in Moore (See Brian Moore, Evaluation of a Dual-Channel Full Dynamic Range Compression System for People with Sensorineural Hearing Loss, Ear and Hearing, Vol. 13, No. 5, 1992, at 349-370) that it is necessary to evaluate improvement by testing in the real world for sustained periods of time. Plomp (See Reinier Plomp, The Negative Effect of Amplitude Compression in Multichannel Hearing Aids in the Light of the Modulation-Transfer Function, J. Acoust. Soc. Am. 83(6), June 1988, at 2322-2327) has suggested that multi-band compression would be detrimental to speech intelligibility because the reduction in dynamic range does not imply a reduction in the size of the just noticeable difference (JND) in amplitude discrimination. Plomp has further suggested that fast time constant compression would lead to reduced amplitude modulation over time, which in turn, would lead to reduced perception of this modulation. It has also been suggested that very fast time constants can create harmonic distortion at low frequencies. The argument was also put forward that fast time constant multi-band compression would reduce spectral contrasts over frequency, thus "whitening" the spectrum, thereby lessening the ability to distinguish vowels. Vilchur (See E. Vilchur, Comments on the Negative Effect of Amplitude Compression in Multichannel Hearing Aids in the Light of the Modulation-Transfer Function, J. Acoust. Soc. Am. 86(1), July 1989, at 425-428) responded to these points. Others have written on related topics. See e.g. L. D. Braida, et al., Review of Recent Research on Multiband Amplitude Compression for the Hearing Impaired, The Vanderbilt Hearing Report, Upper Darby, Pa.: Monographs in Contemporary Audiology, 1982, at 133-140; B. R. Glasberg, et al., Auditory Filter Shapes in Subjects with Unilateral and Bilateral Cochlear Impairments, J. Acoust. Soc. Am. 79, 1986, at 1020-1033; Brian Moore, How Much Do We Gain by Gain Control in Hearing Aids?, Acta Otolaryngol, 1990, 469 Suppl. at 250-256; Igor Nabelek, Performance of Hearing-Impaired Listeners under Various Types of Amplitude Compression, J. Acoust. Soc. Am. 74(3), September 1983, at 776-791; and Walker et al., The Effects of Multichannel Compression/Expansion Amplification on the Intelligibility of Nonsense Syllables in Noise, J. Acoust. Soc. Am. 76(3), September 1984, at 746-757.
Most agree that some form of limiting is required so that loud sounds are not too loud but soft sounds are audible. The debate is focused on full range vs. limiting compression, and on fast vs. slow time constants. Moore (previously cited) suggests that a two or three band compressor, while having sufficient frequency resolution to allow attenuation of low frequency noise and vowel sounds, and while permitting amplification of softer high frequency consonants, is still coarse enough in frequency, as opposed to a critical band compressor, such that spectral whitening will not occur.
Given two input signals of equal energy, one narrow band so that its frequency range is entirely within one critical band, and another wide band so that its frequency range spans several critical bands, the wide band signal will appear louder to the listener. This is due to a psychoacoustic phenomenon called loudness summation. This has implications for compressor design. If the compressor has a few wide bands (e.g. 2), and if the compressor is adjusted such that wide band signals are well matched in loudness to normal loudness growth curves, then narrow band signals will appear too soft. Conversely if the compressor has many independent narrow bands (e.g. critical bands), and if the compressor is adjusted such that narrow band signals are well matched in loudness to normal loudness growth curves, then wide band signals will appear too loud. Hohman (See V. Hohman, Narrow/Wide Band Compensation in Coupled Narrow Band Aid, Reihe 17: Biotechnik, Nr. 93, 1993, at 1-99) has designed a compressor which addresses this problem. It measures not only power but bandwidth of input signals and adjusts gain accordingly. This is called a coupled narrow band compressor.
As illustrated by the above discussion, different signal processing strategies have been developed to address different and specific hearing aid problems. In an attempt to increase the versatility of hearing aids, adjustable hearing aids have been developed. With adjustable hearing aids (which typically employ analog signal processors), certain parameters can be adjusted by the user. By allowing the user to dynamically set the parameters, the adjustable hearing aid allows the user to set the hearing aid to best suit the user's listening environment. While an adjustable hearing aid does impart to the user a greater degree of versatility, this versatility has its limits. Ultimately, an adjustable hearing aid implements the same signal processing strategy, regardless of the parameters. If the particular strategy implemented by the hearing aid is not well-suited for a particular situation, then no amount of parameter adjustment will cause the hearing aid to provide satisfactory results.